Rivers and associated sediments

Rivers and associated
sediments
Weathering and
erosion in
mountain
source area
The hydrologic cycle describes the
continuous movement of water on,
above and below the surface of the
Earth. The water moves from one
reservoir to another, such as from
river to ocean, or from the ocean to
the atmosphere, by the physical
processes of evaporation,
condensation, precipitation,
infiltration, runoff, and subsurface
flow. In so doing, the water goes
through different phases: liquid, solid,
and gas. Rivers are a branch of the
hydrologic cycle.
Transport
downstream
via river
systems
Ultimate deposition
of sediment in
coastal marine
environment
ghiaia/conglomerato > 2 mm
Silt/siltite tra 0.0625 mm e 0.0039
mm (1/256 mm)
Sabbia/arenaria tra 2mm e 0.0625 mm
(1/16 mm)
Argilla/argillite <0.0039 mm
http://www.southampton.ac.uk/~imw/Budleigh-Salterton.htm
Basics. In fluid mechanics, the Reynolds number Re is a dimensionless
number that gives a measure of the ratio of inertial forces to viscous
forces
Re = 2pVL / µ
p is the density of the fluid (kg/m³)
V is the mean velocity of the fluid (SI units: m/s)
L is a characteristic linear dimension (hydraulic diameter of river systems) (m)
μ is the dynamic viscosity of the fluid (Pa·s or N·s/m² or kg/(m·s))
Turbulent water flow occurs at high
Reynolds numbers (>2000): high
V / low μ tends to produce chaotic
eddies, vortices and other flow
instabilities.
Laminar water flow occurs at low
Reynolds numbers (<500): low V /
high μ tends to produce smooth,
constant fluid motion.
Particles of any size may be moved in a fluid by one of three mechanisms.
Rolling: the clasts move by rolling along at the bottom of the water flow without
losing contact with the bed surface.
Saltation: the particles move in a series of jumps, periodically leaving the bed
surface, and carried short distances within the body of the fluid before returning
to the bed again.
Suspension: turbulence within the flow produces sufficient upward motion to
keep particles in the moving fluid more-or-less continually.
At very low water current velocities (very low Re) only fine particles (fine silt
and clay) are kept in suspension. No rolling/saltation of
sand particles.
At low water current velocities (low Re)
fine particles (fine silt and clay)
and low density particles are kept in
suspension while sand-size particles
move by rolling and some saltation.
At higher flow rates (high Re) all
silt and some sand may be kept in
suspension with granules and fine
pebbles saltating and coarser material
rolling.
What causes saltation of grains?
The Bernoulli effect.
Fig.4.3
The Bernoulli effect
The Bernoulli effect:
reduction in pressure
= lift force
Deposition and grading: the Stokes Law
The Hjulstrom diagram shows the relationship between the velocity of a water
flow and the transport of loose grains.The cohesive properties of clay
particles mean that fine-grained sediments require relatively high velocities to
re-erode them once they are deposited, especially once they are compacted.
Arno
alluvione
Po
piena
Po
magra
Depositional environments:
1)  Alluvial fans;
2)  Braided Rivers;
3)  Meandering Rivers;
4)  Deltas;
5)  (Turbidites)
1
2
3
4
5
Alluvial fans are
characterized
by turbulent (high
Re) sheetfloods
occurring during
heavy rainstorms
Sheetflood deposits
consist of
alternations of gravel
and sand layers.
Alluvial fans are
characterized
by high density,
laminar (low Re)
debris flows
Debris flow deposits
are a poorly sorted
admixtures of all
grain sizes
Bedload (braided) rivers
Mid-channel bars in braided systems are typically
characterized by downstream accretion, where
the bar migrates by adding new material to the
downstream end (the upstream end erodes)
Camping near braided rivers is not recommended
Bedload (braided) river deposits
Meandering rivers
Yukon River, Alaska
Val Gardena Sandstone
(Upper Permian, Italy)
Meandering rives
*
*The thalweg
is the line of
fastest flow in
a river.
Meandering rives
Meandering river dynamics
Meandering river
dynamics
Oxbow lake
Meandering rive deposits
Point bar sediments
Laminated sand
gravel
Delta
Environments
Nile Delta
Mississippi Delta
Delta
Delta
Differences in the
grain size of the
sediment supplied
affect the form of a
delta:
(a) a high proportion
of suspended
load results in a
relatively small
mouth bar deposited
from bedload and
extensive delta-front
and prodelta
deposits;
(b) a higher
proportion of bedload
results in a delta with
a higher proportion of
mouth bar gravels
and sands.
Fundamental sedimentological unit of a delta is the
distributary mouth bar, formed where sediment is rapidly
deposited after rivers enters basin
Delta front: Progradation of relatively steep delta
front (1-10°) produces a type of bed geometry called
a clinoform Topset
Foreset
Bottomset
Delta front foresets
Delta Plain
Delta Front
P
Prodelta
Delta Front
Delta front sediments occur as
coarsening-upward succession over
prodelta.
Prodelta (sub-wave base) generally
resembles fine-grained offshore
facies. Slumps may occur due to
steeper prodelta slopes and rapid
sedimentation rate
Wave-Dominated Coasts
In open water, waves are
purely an oscillatory
motion
Where water depth < 0.5
wavelength, water
interacts with bed (=wave
base)
Typical ocean waves have λ = 10-30 m: fairweather wave base (FWWB) is about 5-15 m depth
Large storm waves can have λ < 400 m: storm
wave base (SWB) is anywhere between 30-125+ m
depth
Beach sediments: Low angle
stratification
Not storm-dominated: Wave ripples
Beach sediment: Low angle (<5°), seaward-dipping
upper plane bed stratification, usually well-sorted
mature sand
Wave ripples
Form in oscillatory water motion created by fairweather waves
Symmetrical profile, often sharp crest with “tuningfork” bifurcations
Tidal Flat Environments
Tides are a complex
product of gravitational
attraction (from the moon
and the sun) and Earth’s
rotation. Lunar'dalbulge
rotatesaroundEarthwitha
periodof24hours50minutes
Tidal range is highly variable, depends more local coastline
amplification and development of standing waves (seiches)
Tidal range governs speed of tidal currents during flood
(rising) and ebb (falling) flows
Bidirectional paleocurrent indicators (especially
cross-stratification) are diagnostic of tidal deposition
Herringbone crossstratification
Floodcurrent:'degoingin
Ebbcurrent:'degoingout
Turbidity flows
In deeper water the mouth bar is restricted to an area close to the
river mouth and much of the sediment is deposited by mass-flow
processes in deeper water.
Hypopycnal plume
Hyperpycnal plume
Density < seawater
Density > seawater
Relatively continuous
Episodic, lasts hours-days
Deposition from
suspension
(=hemipelagic
sedimentation)
Deposition from
suspension, modified by
traction
Bouma “Ta” division
Massive normally-graded sandstone reflecting
unhindered settling of particles from waning
Newtonian and turbulent flow
Technically, should always
exhibit normal grading (flow
should be Newtonian, not
plastic)
Parallel laminated sand Tb, rippled sand Tc, faintly laminated
silt Td, and mudstone Te divisions reflect deposition from
waning flow
Bouma Te division partly consists of hemipelagic (nonturbidite) mudstone
E
D
C
B
Due to the Newtonian flow rheology,
turbidites are typically very laterally
extensive, thin beds (typically 5-20
cm)
Turbidity flows can transfer continental sediments from the
continental shelf through canyons to the abyssal plain
Majorfeaturesoftheoceanfloor:
1) Con'nentalShelf–Shallow(lessthan~100m)andrela'velyflatregionoverlyingcon'nentalcrust.
2) Con'nentalSlope—Transi'onregionbetweencon'nentalandoceaniccrust.
3) Con'nentalRise—ThickprismofsedimentdepositedatbaseofCon'nentalSlope.
4) AbyssalPlain—Broadflatplaincoveredwithsediment,overlyingruggedoceanfloor.
5) Mid-oceanRidge—Longcon'nuousridgesrising2-3kmabovethesurroundingseafloorextendingthroughoutall
majoroceanbasins.
6) Deep-seaTrenches—Deep(asmuchas11km)trenchesformedbysubduc'onofoceaniccrust.
Il dissesto idrogeologico (frane e alluvioni) è un problema estremamente
diffuso sul territorio nazionale. Negli ultimi quaranta anni si sono verificati molti
eventi di dissesto idrogeologico che hanno avuto effetti catastrofici. Tra i
principali si ricordano quelli di Firenze (1966), Genova (1970), Ancona (1982),
Val di Fiemme (1985), Valtellina (1987), Piemonte (1994), Versilia (1996),
Sarno (1998), Soverato (2000), Nord-Ovest dell’Italia (2000), Valbruna (2003),
Varenna, Nocera Inferiore (2005), Cassano delle Murge (2005), Ischia (2006),
Vibo Valentia (2006), Messina (2009), Laces (2010).
La crescente incidenza degli
eventi catastrofici corrisponde
ad un progressivo aumento
del rischio idrogeologico
legato all’aumento del territorio
antropizzato spesso in aree
instabili che ha interessato il
territorio nazionale a partire
dal dopoguerra.
Densità di fenomeni
franosi: numero di eventi
rilevati in rapporto alla
superficie territoriale.
A scala nazionale si
registrano 1,56 frane per
kmq.
Superano di molto questo
valore la Lombardia (5,5
frane/kmq), il Molise (5,1
frane/kmq), le Marche
(4,4 frane/kmq) e l’Umbria
(4,1 frane/kmq).
FLOODS
The number of flood disasters by country from
1974 to 2003
•  Distinguish broadly between two kinds of floods: Large
scale river floods, and Flash floods
•  Large scale river floods:
–  Occur in large river basins from large scale heavy rain (Orange,
Vaal, Limpopo, Zambezi Rivers)
–  Long response time (6 hours to many days) between heavy rain
and flooding
–  Complex hydrologic models calculating river levels as the flood
moves down the river over the next few days
•  Flash floods:
–  Occur in small river basins (50 – 200 square km)
–  Quick response (<6 hours) between heavy rain and flooding
–  Traditional hydrologic models ineffective due to small lead
time
Flood mechanisms
•  Hydrometerological floods
- snowmelt runoff
- storm rainfall
- rain-on-snow
- ice jams
•  Natural dam failures
- earth dams (e.g landslides)
- ice dams (e.g jökhalaups)
100
Vancouver I.A.
90
80
Storm
rainfall:
precipitation
and river
response
(October,
2003)
70
60
50
Series1
40
30
20
?
10
0
1
14
152
Q
3
16
174 185
196 207
218 229
Coquitlam R.
at mouth
Understanding hydrological
responses
Discharge m3/s
Peak discharge
Rate of recession
Lag time
Discharge m3/s
Question: what determines the
flood response (lag time, peak
discharge and recession rate) to a
precipitation event?
Rate of recession
Lag time
Discharge
Stream order and flood
response
Discharge
Discharge
Regional
climate
and flood
response
Discharge
Discharge
Discharge
Geological substrate and flood
response
Basin geometry and flood
response
circular
Discharge
Discharge
Discharge
elongated
effect of
storm
path?
Discharge
Discharge
Discharge
Land use and flood response
Summary:
Stream order
Regional climate
Geological substrate
Basin geometry
Land use
Determine the
flood response (lag time, peak
discharge and recession rate) to a
precipitation event.